Knowledge Bank

Measurements of the collision-induced absorption of $O2$ in the vicinity of the electric-dipole-forbidden vibrational fundamental have been made in pure $O2,O2/N2$ and $Ar/O2$ mixtures. The goal is to provide reliable absorption coefficients to allow accurate retrievals of stratospheric concentration profiles of $H2O$ vapor, $NO2$, and sulfate aerosols from limb-viewing filter radiometers, such as those mounted on the UARS and future EOS platforms. Here, spectra were recorded at a resolution of $0.5cm-1$ using a Fourier-transform infrared spectrometer. Measurements were made using an optical pathlength of 84 m, sample densities up to approximately 8 amagats, and sample temperatures between 225 K and 300 K. The present results are in good agreement with the earlier less precise measurements of Orlando et al. on $O2$ and $O2/N2$ mixtures and Thibault et al. on pure $O2$. Studies of $Ar/O2$ mixtures indicate that the observed structure on the high-frequency shoulder of the band, previously attributed to dimer bound states by Henderson and Ewing, is more likely due to line mixing of pure quadrupole transitions of $O2$, as originally suggested by Vigasin.